As compared to austenitic stainless steel, ferritic stainless steel has a higher cost performance in terms of corrosion resistance as well as a better heat thermal conductivity and a smaller coefficient of thermal expansion and is more resistant to stress corrosion cracking. Due to these excellent characteristics, ferritic stainless steel has been used in a wide range of applications including automobile exhaust system components, building materials such as roofs and fittings, and materials used in wet condition such as kitchen furniture, water tanks and hot water tanks.
These structures are most often manufactured by welding stainless steel sheets that have been cut and formed into appropriate shapes. Because ferritic stainless steel has low solid solubility limits of carbon and nitrogen, welding of ferritic stainless steel tends to result in the occurrence of a phenomenon called sensitization in which Cr carbonitride is produced at the weld in the process of the melting and solidification during welding and consequently a Cr depletion layer is formed to cause a decrease in corrosion resistance.
A conventional remedy to this is to add titanium or niobium having higher affinity for carbon and nitrogen than does chromium, thereby suppressing the formation of Cr carbonitride and the occurrence of sensitization. For example, Patent Literature 1 discloses ferritic stainless steel improved in grain boundary corrosion resistance by the combined addition of titanium and niobium.
As the shapes of components that are welded have become more complicated in recent years, sufficient gas shielding during welding is often failed and welding is frequently carried out under such unsatisfactory conditions that atmospheric nitrogen gets mixed with the shielding gas. Under such welding conditions, nitrogen in the shielding gas enters a weld bead to further increase the probability of sensitization at the weld. Thus, difficulties are encountered in ensuring corrosion resistance with conventional ferritic stainless steels disclosed in literature such as Patent Literature 1.
Ferritic stainless steels with excellent weld corrosion resistance have been disclosed. For example, Patent Literature 2 discloses ferritic stainless steel with excellent corrosion resistance at welds, Patent Literature 3 discloses ferritic stainless steel with excellent corrosion resistance at weld gaps, and Patent Literature 4 discloses ferritic stainless steel with excellent corrosion resistance at welds with austenitic stainless steel. Even with these ferritic stainless steels, however, sufficient corrosion resistance cannot be always ensured under such welding conditions that nitrogen will enter from a shielding gas into a weld bead.